1. Field of the Invention
The present invention relates to an indicator for pressure container, attached to a nitrogen gas cylinder for retaining a work in a press working machine by pressing the work against a fixed die, and arranged to give a warning when gas pressure in the work retaining nitrogen gas cylinder decreases below a predetermined value.
2. Description of Related Art
In a press working machine, when press working is to be performed, a work to be worked on is fixed with a nitrogen gas cylinder for retaining the work by downward movement of a movable die. More specifically, a high-pressure nitrogen gas of about 10 MPa is filled into the cylinder and the high pressure gas exerts a constant force on a rod through a piston. For instance, if the cylinder is 120 mm in diameter, a pressure of 10 MPa can apply a force of about 4.4 ton. This nitrogen gas cylinder is placed in a work contact portion of the movable die and configured to bring the rod into contact with the work retainer when the die is moved downward. Accordingly, the work retainer can hold the work with a predetermined force to fix the position of the work. It is therefore possible to enhance the punching accuracy without permitting displacement of the work during a shearing process using a press.
When the movable die is held in direct contact with the work retainer, the pressing force of the movable die directly acts on the work, which may cause deformation of the work. To avoid such disadvantage, the work retainer and the nitrogen gas cylinder are interposed therebetween.
In use, the nitrogen gas cylinder is filled with a high pressure nitrogen gas of about 10 MPa. After repeated use, however, the pressure of the filled nitrogen gas will decrease. When the nitrogen gas pressure decreases below 7 MPa, for example, the pressing force of the work retainer lowers, which is problematic.
To solve the above problem, the nitrogen gas cylinder is conventionally provided with an indicator for pressure container in order to give a warning when the pressure of nitrogen gas in the cylinder decreases to a predetermined value or lower.
When the pressure container indicator is brought in a warning state, an operator has to interrupt a press working operation and refill high pressure nitrogen gas into the nitrogen gas cylinder.
An example of the pressure container indicator is shown in FIG. 19. A hollow case 111 has a cavity 111a opening at a left end, in which a piston rod 112 is slidably fitted. An O-ring 113 serving as a seal member is mounted on the periphery of the piston rod 112. The piston rod 112 is urged leftward by a spring 114. The piston rod 112 also receives, at its left end face, the internal pressure of the nitrogen gas cylinder. A right end of the piston rod 112 forms an indicating portion 112a which can protrude outside the case 111.
When the gas pressure in the nitrogen gas cylinder is a predetermined value or higher, the piston rod 112 is moved rightward against the spring 114 to make the indicating portion 112a protrude outside the case 111. When the gas pressure in the nitrogen gas cylinder becomes lower than the pressure value, on the other hand, the force of the spring 114 exceeds the force of the gas pressure. Accordingly, the piston rod 112 is moved rightward to retract the indicating portion 112a into the case 111.
Retracting of the indicating portion 112a makes the operator who operates the press working machine recognize that the gas pressure in the nitrogen gas cylinder is abnormal.
However, the conventional abnormality indicator has the following disadvantages. The piston rod 112 moves in proportion to the gas pressure in the nitrogen cylinder. When the gas pressure is around the predetermined value, the position of the indicating portion 112a is unclear, which makes it difficult to determine whether the gas pressure is normal or abnormal. Further, the indicating portion 112 is retracted under abnormal condition. This is hard to visually recognize.
A technique for solving the above problems has been proposed. For example, FIG. 20 shows an indicator for pressure container disclosed in Japanese patent No. 2843491. A protrusion 105 is provided on an end face of bellows 104 communicating with the gas of a pressure container. Under normal condition, the protrusion 105 is engaged with a rotating lever 106 which is engaged with a stopper 103. The stopper 103 serves to stop an indicating rod 101 within the case under normal condition. The indicating rod 101 is urged by a spring 102 in a direction to protrude from the case.
When the gas decreases to the predetermined value or lower, the position of the bellows 104 containing gas moves to the left, causing a positional change of the protrusion 105 of the bellows 104. The rotating lever 106 is thus rotated, disengaging the stopper 103 from the indicating rod 101. The indicating rod 101 is then allowed to protrude outside the case by the urging force of the spring 102.
Specifically, there is disclosed an indicator with the indicating rod 101 which can be disengaged and protruded by the spring 102.
Here, the bellows, which changes its position while internally containing gas, is unlikely to cause gas leakage even without using any special sealing mechanism, and thus is convenient.